A BS TRACT: The synuclein family includes three neuronal proteins, named α-synuclein, β-synuclein, and γ-synuclein, that have peculiar structural features. α-synuclein is largely known for being a key protein in the pathophysiology of Parkinson's disease (PD) and other synucleinopathies, namely, dementia with Lewy bodies and multisystem atrophy. The role of β-synuclein and γ-synuclein is less well understood in terms of physiological functions and potential contribution to human diseases. α-synuclein has been investigated extensively in both cerebrospinal fluid (CSF) and blood as a potential biomarker for synucleinopathies. Recently, great attention has been also paid to β-synuclein, whose CSF and blood levels seem to reflect synaptic damage and neurodegeneration independent of the presence of synucleinopathy. In this review, we aim to provide an overview on the pathophysiological roles of the synucleins. Because γ-synuclein has been poorly investigated in the field of synucleinopathy and its pathophysiological roles are far from being clear, we focus on the interactions between α-synuclein and β-synuclein in PD. We also discuss the role of α-synuclein and β-synuclein as potential biomarkers to improve the diagnostic characterization of synucleinopathies, thus highlighting their potential application in clinical trials for disease-modifying therapies.

Introductionβ-synuclein (β-syn) is a presynaptic protein, whose cerebrospinal fluid (CSF) levels are increased in patients with Alzheimer’s diseases (AD) showing mild cognitive impairment (MCI) and dementia (dem). Here, we aimed to investigate CSF β-syn in subjects at different AD stages, including preclinical AD (pre-AD), and to compare its behaviour with another synaptic biomarker, α-synuclein (α-syn), and two biomarkers of neuro-axonal damage, namely neurofilament light chain protein (NfL) and total tau protein (t-tau).MethodsWe measured β-syn, α-syn, t-tau and NfL in CSF of 75 patients with AD (pre-AD n=17, MCI-AD n=28, dem-AD n=30) and 35 controls (subjective memory complaints, SMC-Ctrl n=13, non-degenerative neurological disorders, Dis-Ctrl n=22).ResultsCSF β-syn, α-syn, t-tau were significantly elevated in pre-AD patients compared with controls (p<0.0001, p=0.02 and p=0.0001, respectively), while NfL only increased in dem-AD (p=0.001). Pre-AD cases showed lower t-tau concentrations than MCI-AD (p=0.04) and dem-AD (p=0.01). CSF β-syn had the best diagnostic performance for the discrimination of pre-AD subjects from all controls (area under the curve, AUC=0.97) and from SMC-Ctrl subjects (AUC=0.99).DiscussionCSF β-syn increases in the whole AD continuum since the preclinical stage and represents a promising biomarker of synaptic damage in AD.